1. Field of the Invention
The present invention relates to a light emitting device driving circuit, and particularly relates to a light emitting device driving circuit, which has a compensation mechanism.
2. Description of the Prior Art
In a conventional driving circuit for a light emitting device (for example, a light emitting diode), a PWM (Pulse Width Modulation) circuit is utilized to control a switch device for controlling the current flowing through the light emitting device. Also, a comparator is utilized to compare a reference voltage with a feedback voltage proportional to the current to determine if the current should decrease or increase. Such a technique is a well-known controlling technique called peak-current controlling technique. This technique has many disadvantages, however. For example, the current ripple based on this structure substantially varies if input voltage varies and the input voltage of the light emitting device may be unfortunately an AC voltage, so the current accuracy control and the endurance of the light emitting device may be negatively affected. Moreover, the PWM circuit operates at a fixed frequency, such that the circuit will cause stronger electromagnetic interference to other devices. Besides, an oscillation may occur if the switch device is accidentally turned on when it is expected to be off.
FIG. 1 is a circuit diagram illustrating a driving circuit of prior art for a light emitting device. As shown in FIG. 1, the light emitting device driving circuit 100 includes a comparator 101, a driving module 103 and a time counting circuit 105. The comparator 101 compares a feedback voltage Vfb (which is generated according to the resistor 108 and a driving current I) and a reference voltage Vref, and controls the driving module 103 to output a control signal VG for controlling the switch device 107, thereby controlling the current I flowing through the light emitting device 109 and the inductor 111. The light emitting device driving circuit 100 further includes a time counting circuit 105 that controls the driving module 103 such that the switch device 107 will be turned on again after being turned off for a predetermined period of time. The resistor 106 is used to adjust the predetermined period of time, and such a technique is a well-known skill called constant-off time.
The advantage of this method is that, since the turn off time is constant, the range of the current ripple will not change as input voltage changes, and the problem of poor current accuracy may be avoided. Also, since the operation frequency of the PWM circuit in FIG. 1 is not fixed, the circuit will cause less electromagnetic interference problem. Furthermore, since the decreasing rate of the driving current is almost fixed, switch device 107 may not be accidentally turned on, avoiding the above-mentioned problem of oscillation.
However, the structure in FIG. 1 may have some problems. FIG. 2 illustrates a current-time relation of the driving circuit of prior art for a light emitting device. As shown in FIG. 2, the maximum value of the current I is originally set to be ISU. In view of signal propagation, the circuit in FIG. 1 always has a delay time starting at the time when the feedback voltage Vfb reaches the reference voltage Vref and ending at the time when the control signal VG really turns off switch device 107. Thus, during the delay time, current I will exceed the originally-set ISU and finally reach Imax. As mentioned before, the turn off time toff is set as a constant such that Imin is a function of Imax. Thus, the real mean driving current (=(Imax+Imin)/2) will exceed the expected mean driving current Imean (˜(Imax+Imin)/2). The difference between the real and expected mean driving currents implies driving current inaccuracy. The larger the input voltage Vin, the more significant the driving current inaccuracy.